Elsa Lanchester plays both Mary Shelley and the Bride.
In addition to her role as Minnie, Una O'Connor also appeared in the prologue, as Shelley's maid who is holding the leash as the dogs go off screen.
Deleted Scene: James Whale cut out a few scenes before release, trimming the movie down from 90 minutes to 75; this included a re-shot ending where Dr. Frankenstein survives (he was originally meant to die in the laboratory explosion) and some bits that were excised at the behest of the censors, like a scene where Dwight Frye's character murders his uncle and blames it on the monster, or some shots of Elsa Lanchester that were deemed too revealing. Don't expect to ever see these scenes.
Executive Meddling: Due to the Hays Code now being fully in effect, they were now more limited on the violence and number of deaths they could show.
Leaning on the Fourth Wall: The final note of the musical score, played just as the laboratory explodes, is a very powerful dissonant chord, made to be so loud that it would slightly shake the seats in the movie theater. Whale came up with this idea, and meant for it to suggest that the explosion was so massive that it even affected the real world. This element is, unfortunately, mostly lost when watching it on home video.
Not Named in Opening Credits: The opening credits bill "The Monster's Mate" with a question mark, and only gives Elsa Lanchester credit for playing Mary Shelley in the prologue. This also acts as a sort of Call Back to how Karloff was credited in the first movie.
The Other Darrin: The actors for Elizabeth, the Burgomaster, and Hans are replaced.note Elizabeth was played by Mae Clarke in the original Frankenstein and by Valerie Hobson in Bride of Frankenstein due to Clarke's illness at the time. This is a bit jarring since Bride picks up right where the original leaves off and Elizabeth's hair changes from blonde to brunette.
Science Marches On: "The human heart is more complex than any other part of the body". Apparently growing brains in a vat is easier than we think.
Stock Footage: Footage from the first film is shown while Lord Byron is remembering the story.
What Could Have Been: Original plans for a sequel to Frankenstein were very different. Two other possible plots were conceived, one being the Monster continuing Dr. Frankenstein's research and the other one being Dr. Frankenstein inventing a death ray in the eve of World War I.
The role of Dr. Pretorius was offered to Claude Rains, but he was attached to another project at the time and couldn't participate. They also offered it to Bela Lugosi, but he turned it down.
In the film's original ending, Elizabeth was supposed to be killed and have her heart placed in the Bride (hence why the Bride seems so drawn to Henry during the scene) and Henry was supposed to die in the explosion as well (he can still be seen pinned against the wall in the final cut).
As revealed on Svengoolie, the late midget actor Billy Barty played a baby in Pretorius' collection of little people. His role was cut from the final film, but he can be seen from behind in a wide shot of the collection of little people in jars.